The present invention relates to a semiconductor memory device and, more particularly to, a non-volatile semiconductor memory device such as a NAND cell-, NOR cell-, DINOR cell-, or AND cell-type EEPROM.
Conventionally, an electrically rewritable EEPROM is known as one of the semiconductor memory devices. Among others, a NAND cell-type EEPROM in which each NAND cell block is made up of a plurality of memory cells connected in series is attracting attention as a device that can have a high degree of integration.
Each memory cell of a NAND cell-type EEPROM has a FET-MOS structure in which a floating gate (charge storage layer) and a control gate are stacked with an insulating film there between on a semiconductor substrate. A plurality of adjacent memory cells share sources and drains and are connected in series to thereby make up a NAND cell, which is connected to a bit line as a unit. Such NAND cells are arranged in a matrix, thus constituting a memory array. The memory array is integrally formed in a p-type semiconductor substrate or in a p-type well.
Each drain positioned at one end of the NAND cells connected in series in a column direction of the memory cell array is commonly connected via a select gate transistor to a bit line, while each source positioned at the other end is also connected via a select gate transistor to a common source line. The control gates of the memory transistors and the gate electrode of the select gate transistors are commonly connected respectively as a control gate line (word line) and a select gate line in the row direction of the memory cell array.
This NAND cell-type EEPROM operates as follows. Data programming operations mainly start from a memory cell which is the most remote from the bit line contact. First, when the data programming operation starts, according to write-in data, the bit line is given 0V (for “1” data write-in bit line) or a power supply voltage Vcc (for “0” data write-in bit line) and the select gate line on the side of a selected bit line contact is given Vcc. In this case, in a selected NAND cell connected to the “1” data write-in bit line, its channel portion is fixed to 0V by way of a select gate transistor. In a selected NAND cell connected to the “0” data write-in bit line, on the other hand, its channel portion is charged via the select gate transistor up to [Vcc−Vtsg] (where Vtsg is a threshold voltage of the select gate transistor) and then enters a floating state. Subsequently, one control gate line in the selected memory cell in the selected NAND cell changes in potential from 0V to Vpp (=20V or so, which is a programming high voltage), while the other control gate line in the selected NAND cell changes in potential from 0V to Vmg (=10V or so, which is an intermediate voltage).
Since a selected NAND cell connected to the “1” data has a large potential difference (=20V or so) between its selected memory cell's control gate line (=Vpp potential) and its channel portion (=0V), thus causing electrons to be injected from the channel portion to the floating gate. Accordingly, the threshold voltage of that selected memory cell shifts to the positive direction, thus completing write-in of data “1”.
A selected NAND cell connected to the “0” data write-in bit line, on the other hand, has its channel portion in a floating state, so that an influence of capacitive coupling between its control gate line and its channel portion raises a voltage of the control gate line (0V·Vpp, Vmg), which in turn raises a potential of the channel portion from a [Vcc−Vtsg] potential to Vmch (=8V or so) with that channel portion as held in the floating state. In this case, since a potential difference between the control gate line (=Vpp potential) and the channel portion (=Vmch) of the selected memory cell in the selected NAND cell is a relatively low value of 12V or so, thus electron injection is avoided. Therefore, the threshold voltage of the selected memory cell is held unchanged at a negative value.
Data erase is carried out to all of the memory cells in a selected NAND cell block. That is, 0V is applied to all the control gate lines of the selected NAND cell block, while a high voltage of 20V or so is applied to the bit lines, the source lines, the p-type well regions (or p-type semiconductor substrate), and the control gate lines and all the select gate lines in the non-selected NAND cell blocks. Thus, in all the memory cells in the selected NAND cell block, the electrons in the floating gate are emitted to the p-type well (or the p-type semiconductor substrate), thus shifting the threshold voltage to the negative direction.
Data read-out, on the other hand, is carried out by applying 0V to the control gate line of a selected memory cell and a read-out intermediate voltage Vread (4V or so) to the control gate line and the select gate line of the other memory cells to thereby detect whether a current flows through that selected memory cell.
As may be obvious from the above description, to write data into a NAND cell-type EEPROM, it is necessary to apply voltages higher than the power supply voltage, i.e. Vpp (20V or so) to a selected control gate line in a selected block and Vmg (10V or so) to a non-selected control gate line in that selected block.
To apply the above-mentioned voltages Vpp and Vmg, in a row decoder circuit, the current paths of two kinds of elements of an NMOS transistor (n-channel type MOS transistor) and a PMOS transistor (p-channel type MOS transistor) having different polarities are connected in parallel to the control gate line to conduct control so that both transistors may be turned ON and OFF in a selected block and in a non-selected block respectively.
FIG. 1 is a circuit diagram for showing a configuration example of part of the row decoder circuit in such a conventional semiconductor memory device.
In the circuit shown in FIG. 1, connected to each control gate line are one NMOS transistor (Qn1 to Qn8)+one PMOS transistor (Qp1 to Qp8). Those transistors Qn1 to Qn8 and Qp1 to Qp8 are supplied with complementary control signals from nodes N1 and N2 respectively.
For data write-in, the power supply node VPPRW and a selected control gate line have the same level in voltage like power supply node VPPRW=[selected control gate line voltage]=20V. In this case, connected to each control gate line are one NMOS transistor+one PMOS transistor, so that 20V can be applied to the control gate line even when the power supply node VPPRW is 20V. Accordingly, it is not necessary to raise the power supply node VPPRW to (20V+Vtn) in order to apply both voltages of 0V and Vpp in a selected block.
Note here that in the circuit shown in FIG. 1, memory cells M1 to M8 have their current paths connected in series, thus making up one NAND cell. One end of the each NAND cell is connected via the current path of the select gate transistor S1 to the bit lines BL1 to BLm and the other end, via the current path of the select gate transistor S2 to the source line (Cell-Source) commonly. The control gate lines CG(1) to CG(8) are commonly connected to the control gates of the memory cells M1 to M8 respectively in each NAND cell, while the select gate lines SG(1) and SG(2) are commonly connected to the gates of the select gate transistors S1 and S2 respectively. The signal input nodes CGD1 to CGD8, SGD, SGS, and SGDS are each supplied with a decode signal. Moreover, the row decoder activating signal RDEC is at Vcc during general data programming, read-out, and erase and at 0V during non-operation. The block address signal RA1, RA2, and RA3 are all at Vcc in a selected block and at least one of them is at 0V in the non-selected blocks.
All the PMOS transistors arranged in a region HV indicated by a broken line in the figure are formed in the n-well region to which the programming high voltage Vpp is applied, so that either of the nodes N1 and N2 is always at Vpp during write-in. Furthermore, the node SGDS is at 0V during write-in.
By the above-mentioned configuration, however, each of the control gate lines CG(1) to CG(8) requires two transistors Qp1 to Qp8, Qn1 to Qn8 to thereby increase the number of elements hence a pattern occupied area in the row decoder circuit, thus problematically raising the chip cost.
To prevent an increase in the number of the elements in the row decoder circuit, on the other hand, such a circuit as shown in FIG. 2 may be used in which one transistor (e.g., only NMOS transistor QN1 to QN8) is connected to each control gate line. The circuit shown in FIG. 2 has almost the same configuration of a memory block 2 as that of FIG. 1 but is different therefrom in the circuit configuration of parts 5a and 5b of the row decoder circuit (control gate lines CG(1) to CG(8) and a transistor portion for applying voltages to the select gate transistors S1 and S2 and in that a pump circuit PUMP is provided.
In a case of this circuit configuration, to apply the programming high voltage Vpp to the control gate lines CG(1) to CG(8), it is necessary to apply [VPP+Vtn] to the gates of the NMOS transistors QN1 to QN8 connected to these control gate lines CG(1) to CG(8), where Vtn is a threshold voltage of the NMOS transistors QN1 to QN8 connected to the control gate lines CG(1) to CG(8). Therefore, the pump circuit PUMP is provided in the row decoder circuit.
This pump circuit PUMP comprises capacitors C1 and C2, NMOS transistors QN21 to QN23, an inverter 6, a NAND gate 7, and depletion-type NMOS transistors QN24 and QN25.
In the circuit shown in FIG. 2, a signal OSCRD acts as an oscillation signal during data write-in and read-out, so that a voltage raised in the pump circuit PUMP is output to a node N1 and applied along the current paths of the transistors QN1 to QN8 to the control gate lines CG(1) to CG(8). A signal TRAN is constantly set at 0V.
The above-mentioned pump circuit PUMP has the plurality of capacitors C1 and C2 and so has a large area. Those two capacitors C1 and C2, in particular, usually occupy a larger pattern area than any other elements, thus leading to a problem that the pattern area of the row decoder circuit cannot sufficiently be reduced although the number of the transistors required for applying voltage can be decreased.
Thus, the conventional NAND cell-type EEPROM needs to provide a function for sending a high voltage to the word lines to thus require a plurality of transistors to each word line in the row decoder circuit. This leads to a problem of an increase in the pattern area of the row decoder circuit.
If, to solve this problem, one transistor is connected to each word line in the row decoder circuit, the row decoder circuit needs to have a pump circuit therein, a large pattern area of which pump circuit still increases the pattern area of the row decoder circuit.
Further, if the row decoder has one transistor connected to each word line and has no pump circuit therein, the programming high voltage cannot be applied to the word lines without a drop in potential, thus giving rise to a risk that data may not securely written in.